also rod length plays a part as well in the stroke

piston height can be a factor in the overall stroke that can be used within an engine as a shorter length between the rod pin and the crown will enable you to have a slightly larger throw or longer rod compaired to an identical engine with a larger pin to crown length as it would protrude out of the bore if it were the same length. But as said the height of the piston is mainly to do with the CR ratios.

the larger the capacity the more torque and power that can be obtained from the same mods as the increased capacity offers the chance for more air to enter the cylinders and in turn more fuel creating a lsrger bang within the cylinder (i said chance as at full throttle with the engbine approaching the red line the time the valve is open for gets les and less and there is a critical point where the cylinder can no longer obtain enough ai to be full in the time the valve is open, this is normally at the point of peak power)

if you were going to change to a 2ltr for the cost of the calibra pistons and also the work required to fit them to the rods then i would just get some propper pistons for zetec (likes of JE are cheap)